1. Field of the Invention
This invention relates in general to the field of position measurement and more particularly to an improved apparatus and method of providing position-related information.
2. Description of Related Art
A variety of techniques are known in the art to measure position, including land surveying techniques and global positioning satellite (xe2x80x9cGPSxe2x80x9d) system techniques.
Many of these techniques are limited by high cost apparatus often due to the complexity of manufacturing complex devices with high precision and accuracy. Additionally, many of these techniques required extensive training, and therefore are not usable by those not trained in the art.
It is an object of the present invention to provide an improved low cost optical transmitter which overcomes many of the problems associated with prior art position measurement systems.
The system described in this disclosure contains, at a high level, several transmitters and a receiving instrument. The transmitters transmit signals from their stationary locations and the receivers receive these signals. In the present system, laser beams and LEDs are used as the signals. The receiving instrument then determines a coordinate system and calculates its position and assorted other information of interest from these received signals. The receiving instrument then displays this information through a user interface. The information may be, for example, the location of the receiving instrument or its distance relative to another location.
As is clear from the present disclosure, the present invention can be applied to a variety of different fields, applications, industries, and technologies. The present invention can be used, without limitation, with any system in which information related to position must be determined, including without limitation movement, dimensional measurement, and position and orientation tracking. This includes without limitation many different processes and applications involved in myriad industries. Some of these industries and some of their associated processes or applications are: film making (digitizing models, virtual sets, camera tracking, automatic focusing), construction (trades, power tools, surveying, CAD, equipment control, construction measurement and layout), robotics (robot calibration, work cell configuration, mobile robot navigation, hazardous waste excavation), law enforcement (accident scene mapping, crime scene mapping, incident reconstruction), computers (3D input devices, video games), virtual reality (augmented reality, virtual arcades, 3D Internet experiences), manufacturing (factory automation, facility decommissioning, parts inspection, facility maintenance, manufacturing tooling and inspection, manufacturing measurement), medical (surgical navigation, smart operating rooms, medical instrumentation), and defense (ordnance modeling, simulation training, aircraft fit-checks, ship retrofit and repair, site remediation).
Various figures are included throughout this disclosure to illustrate a variety of concepts, components of several subsystems, manufacturing processes, and assembly of several subsystems.
1. Transmitter
The present invention can be used in conjunction with the techniques and apparatus described in co-pending provisional patent application U.S. Ser. No. 60/104,115 to Pratt, also assigned to the present assignee, filed on Oct. 13, 1998, and incorporated herein by reference. The following description in this section is intended to highlight certain features of the incorporated provisional. Certain attached figures, including the xe2x80x9cRotor/Bearing Housing Assemblyxe2x80x9d figure, the xe2x80x9cLaser Assemblyxe2x80x9d figure, and the xe2x80x9cAsymmetric Pulse Effectxe2x80x9d figure, provide additional detail. Additional figures also depict various (i) components of a transmitter, (ii) manufacturing processes for parts of a transmitter, and (iii) operational concepts, including beam fanning, tracking, and mathematics, which are related to a transmitter
A. Simplified Optical Path
As is clear from the incorporated application, one of the key advantages of the Arc Second transmitters is the simplification of the optical paths as exemplified by the lasers rotating with the head. Additionally, there is no window in the preferred transmitter. Therefore, there is no distortion introduced by the movement of the laser beam across a window. As described, the preferred embodiment utilizes a lens or other device which rotates with the laser. Thus, there is no distortion caused, for example, by variable window characteristics or angles of incidence or between a rotating lens and a fixed laser. The absence of a fixed window also simplifies manufacture, maintenance, and operation. The absence of a fixed window does require that a rotating seal be added to the transmitter.
B. Speed of Rotation and Storage of Parameters
As is also described in the incorporated patent application, the rotating head, and the lasers within it, rotate through a full 360 degrees at a constant, although configurable, velocity. Having an easily quantifiable center of rotation simplifies the algorithms for determining position and can simplify the set-up of the system. This determination is also simplified by the utilization of the synchronization signal which fires, in the preferred embodiment, once per revolution of the rotating head. For accuracy in a position measurement system, the angular velocity of the rotating head must not change during each revolution of the head.
The velocity of the rotating head is configurable through the use of, in the preferred embodiment, a field programmable gate array (xe2x80x9cFPGAxe2x80x9d). Such configurable speed control allows the transmitters to be differentiated by a receiver based on the transmitters"" speed of rotation. The use of multiple transmitters, as is appreciated by those of ordinary skill in the art, enhances position detection. Other advantages are obtained through the use of programmable electronics (FPGAs, flash memory, etc). Not only can the desired speed be set by changing the clock to the phase locked loop which controls motor speed, but the overall gain of the control loop can be programmed to maximize performance at the velocity of interest.
C. Beam Type and Number
As described in the incorporated provisional and known in the art, position detection is also enhanced by using multiple beams and controlling the shape of those beams. These beams may be in the same rotating head assembly or in separate rotating head assemblies.
Two beams is the preferred number per rotating head assembly, however, more beams can be used. In particular, another embodiment uses four beams, two for short range and two for long range. The two short-range beams should have fan angles as large as possible. This allows the user to operate near the transmitters, such as in a room. For long-range, the user would normally be operating away from the transmitters. Therefore, in that circumstance the vertical extent of the beams could be reduced to maximize the range of the system. The beams are, preferably, of type III laser. However, the rotation of the beams reduces their intensity to the fixed observer such that they can be classified as type I lasers. Safety features are integrated into the device to prevent the powering of the lasers when the rotating head is not in motion. In the preferred embodiment at least two interlocks are utilized. The first depends on the phase lock loop. The lasers are turned off until the system is in phase-lock for at least 1024 phase-clock-cycles (approximately 32 revolutions). The second is monitoring the absolute speed using the once-per-rev index on the encoder. A tolerance is programmed into the system, currently 1-part-in-1000. When the velocity is outside that window the laser is not allowed to operate.
D. Beam Shape
The Transmitter allows flexibility in setting beams for the application. One advantage is that the beam shape can be modified for the application. The key is that the beam shape should correspond with correctly filling the desired work volume. For construction trades this might be a room 20 mxc3x9720 mxc3x975 m in size. For construction machine control this might be a space 100 mxc3x97100 mxc3x9710m in size. By modifying the beam shape, the energy can be properly directed.
The beam shape can also be controlled to differentiate beams. This can be done for multiple beams on a given transmitter or on different transmitters. For a given transmitter, the first and second beams must be differentiated. One technique uses their relative position with respect to the strobe in time. Another technique is to assure that the beams have different widths (xe2x80x9cbeam widthxe2x80x9d or xe2x80x9cdivergence anglexe2x80x9d). Then, for instance, the first beam could be the xe2x80x9clargerxe2x80x9d of the two beams.
Fanning the beam can be done using a variety of methods known in the art, including without limitation, rod lenses, pal lenses, and cylindrical lenses. The use of rod lenses offers a relatively simple approach, whereas the use of pal lenses offers greater control over the energy distribution. The beam typically is emitted from the source as a conical beam, then a collimating lens shapes the beam into a column, then the fanning lens fans the column.
Rod lenses can be used to increase control on divergence. One of the major advantages of rod lenses for line generation is that they do not directly affect the quality of the beam in the measurement direction (beam direction). Therefore, they should not affect the divergence of the laser beam as set by the collimating optics.
Pal lenses can be used to increase control of the energy distribution in the fan direction. PAL type lenses can even create xe2x80x9cuniformxe2x80x9d distributions, where the energy is uniform in the direction of the fan plane. A uniform distribution is often inefficient, however, if potential receivers are not uniformly distributed along the entire fan plane. In some implementations a focus must be created before the lens. In that implementation, the use of the PAL technique could affect the beam in the measurement direction.
Gaussian beams can also be used to maximize the performance of the receiver. Gaussian beams are symmetric beams in that the energy distribution across the divergence angle or beam width is symmetric. When a simple threshold technique is used in the receiver, it important that the pulses be symmetric and be without shoulders or sidelobes. It is also helpful if the distribution""s shape does not change with range. There are several pulse shapes that meet many of these criteria. However, the Gaussian distribution meets all of these criteria. With symmetric pulses that do not have shoulders or sidelobes, the receiver will be able to detect the center of the beam. Non-symmetric pulses, conversely, can cause the receiver to falsely identify the exact time when the beam center intersects the receiver""s optical detector.
E. Strobe
In a disclosed embodiment, the strobe pulse must be symmetric and pulse shaping in the flash/strobe pulse generator is required. With a simple thresholding technique, it is important that the strobe pulse be symmetric. A square pulse with equal rise and fall times is one desired pulse shape. The light output of the LEDs is directly to the current flowing through the LEDs. Because of the high currents involved in creating the strobe pulse, a pulse-forming network must be used to assure that the current is a square wave as it passes through the diodes. The ideal strobe pulse produces in the optical detector of the receiver a pulse shape identical to a laser pulse.
F. Communications and Control
A disclosed system uses a serial port for communication and control. This allows calibration data and control parameters to easily be transferred. Recall that the transmitters are differentiated by their speeds. Therefore a technique must be put in place to simplify the speed changes. Additionally, the transmitter parameters must be made available to the receiver. To create a simple, reliable, and unified technique the preferred embodiment uses serial communication between the transmitter and the receiver or test equipment. For test purposes, the serial connection is a well-known RS-232 connection. For used in the field, the connection is through an infrared serial port. This allows the transmitter to be sealed and yet communicate with the outside world. To avoid interference with the measurement technique, this port is only active when the lasers are off.
G. VHDL
Many of the digital designs of a disclosed embodiment are implemented in field programmable gate arrays (FPGAs). These devices allow complex designs to be programmed into general-purpose hardware available from multiple vendors. The programs for these devices are written in a special computer understandable language VHDL (VHSIC [very high-speed integrated circuit] Hardware Description Language). This is the same language that is used to design microprocessors and other semiconductor devices and is now standardized as IEEE 1076.
H. Providing Power to the Laser Head
As explained in the incorporated provisional application, the motor and the provision of power to the rotating head assembly are key components of a transmitter according to the preferred embodiment.
A rotary transformer is used. Several techniques are available for powering devices in a rotating head. The most common is the use of slip rings. Unfortunately, slip rings require physical contact between the xe2x80x9cbrushesxe2x80x9d and the xe2x80x9cslip-ringxe2x80x9d. This creates dust in the system and can cause variations in motor speed a frictional torque varies. The preferred technique is to use a rotating transformer. The transformer technique causes minimal drag on the motor. Additionally, through the use of flat signal transformers as power transformers, the technique is very compact.
Fly-back control is used on the stator side of the transformer. To minimize the number of components in the rotating head, the voltage control is performed on the stator side of the transform. To optimize efficiency, a fly-back driving technique is utilized.
I. Stability and Precision of Rotation
The stability of the speed control system and drive motor is also discussed in the incorporated application. As those of ordinary skill in the art will recognize, a sine wave drive motor is a low-cost motor with good inherent stability intra-revolution and, as such, is useful in ensuring constant velocity rotation.
The bearing separation should be maximized to achieve optimal results. Any precession and wobble (wow and flutter in a turntable) will be a source of error in the system. It will lead directly to an error in the xe2x80x9czxe2x80x9d direction. Using two precision bearings and maximizing the distance between the bearings can minimize these errors.
The strobe pulses are triggered by a once-per-revolution indicator tied to the motor shaft. There are many ways to create this shaft position index. The simplest and preferred technique is to use the index normally supplied with an optical encoder. This separate output of the encoder is directly equivalent to a shaft position index.
The optical encoder disk is used to give rotation information. Other devices, including without limitation, tachometers and synchros could be used. The optical encoder disk is typically made of glass and has a series of radial marks on it which are detected as the disk rotates. Additionally, the disk typically has a single xe2x80x9cindexxe2x80x9d mark of a different radius which is used to detect complete rotations. The disk system produces a square wave with a frequency dictated by the speed with which the radial marks are passing. For example, if the disk is rotating at 1 revolution/second, a 1000 mark disk system would produce a 1000 Hz square wave (1000 radial marks/revolution*1 revolution/second=1000 Hz).
The speed of the motor is controlled through a feedback phase-locked loop (PLL) system. The disk system square wave is one input and a clock from the transmitter system is the other input. The transmitter clock has a selectable frequency. The output of the PLL is used to control the speed of the motor rotation such that the PLL remains locked at the selected frequency.
The index mark of the disk can also be used to initiate the strobe pulse as often as once/revolution.
J. Low Manufacture Cost
As more fully described in herein after regarding the calibration facility, a transmitter needs to be stable. Further, it is important for the receiving instrument to know, with precision, the operating parameters of the transmitter. The present manufacturing process for the transmitter specifies these operating parameters with less precision than that required by the receiving instruments. This allows the manufacturing process to be relatively inexpensive. The required precision is obtained in the characterization process, which utilizes the calibration facility described below.
2. Calibration
As discussed below the calibration facility explains the process of determining several key parameters of the transmitter. These parameters need to be provided to the receiving instrument in order to allow the receiving instrument to make the position calculations. Preferably, these parameters are stored in memory in the position calculation engine (PCE) and can be updated as required. For example, if a new transmitter is added to the system, then a new set of parameters needs to be loaded into the PCE. As an additional example, if the rotation speed of a transmitter is changed, then this information needs to be updated in the PCE.
3. Receiving Instrument
In the present system, the preferred receiving instrument is a wand, which is shown in FIG. 18A known as a Walk-About Receiver, and one end of the wand contains a hand-held gun-shaped unit called the Vulcan Receiver, as shown in the FIG. 19. Both of these instruments in earlier versions were commercially available from Arc Second, Inc. of Dulles Va. The wand preferably contains two detectors/receivers.
In the xe2x80x9cVulcan Receiverxe2x80x9d FIG. 18, there is shown a Position Calculation Engine (xe2x80x9cPCExe2x80x9d) which performs most of the computations of the receiving instrument. The PCE as herein after explained is integral in the set-up procedure, tracking, position calculation, and information display.
The Smart Tip can also perform computations, as indicated by the FPGA (field-programmable gate array) and the xe2x80x9ci Buttonxe2x80x9d in each Smart Tip. The Smart Tip can be present at either end in the present system and the signal xe2x80x9cTip Presentxe2x80x9d indicates whether there is a Smart Tip on each of the ends.
The wand provides a lightweight, mobile receiving instrument. Herein after there is a more detailed description of the operation of the wand as well as its configuration and the determination of the location of the wand tip.
4. Position Measurement Systems
As hereinafter explained, the mathematical description of the transmitter and its use in position determination are more fully explained and is a function performed primarily in the PCE.
In accordance with an aspect of the present invention, the functionality disclosed herein can be implemented by hardware, software, and/or a combination of both. Software implementations can be written in any suitable language, including without limitation high-level programming languages such as C++, mid-level and low-level languages, assembly languages, and application-specific or device-specific languages. Such software can run on a general purpose computer such as a 486 or a Pentium, an application specific piece of hardware, or other suitable device. In addition to using discrete hardware components in a logic circuit, the required logic may also be performed by an application specific integrated circuit (xe2x80x9cASICxe2x80x9d), a programmed programmable logic device (xe2x80x9cPLDxe2x80x9d), or other device. The system will also include various hardware components which are well known in the art, such as connectors, cables, and the like. Moreover, at least part of this functionality may be embodied in computer readable media (also referred to as computer program products), such as magnetic, magnetic-optical, and optical media, used in programming an information-processing apparatus to perform in accordance with the invention. This functionality also may be embodied in computer readable media, or computer program products, such as a transmitted waveform to be used in transmitting the information or functionality.
The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing disclosure. The invention is not to be construed as limited to the particular forms disclosed, because these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit and scope of the invention.